Stents are generally tubular shaped devices that function to hold open a segment of a blood vessel or other anatomical lumen and are useful in the treatment of atherosclerotic stenoses in blood vessels. A wide variety of stents have been developed for treating diseases of the blood vessels and other tubular structures inside the body. Stents are particularly suitable for use in supporting and holding back a dissected arterial lining that can occlude the fluid passageway therethrough.
In order to accomplish precise placement of stents within a corporeal lumen and to ensure that a stent placed at a particular position within a body remains at that position, various means are employed. The currently available stents fall within two broad categories: balloon expandable and self-expanding.
A balloon expandable stent is attached onto the outside of an uninflated balloon, which is then introduced into the body vessel and expanded at the desired stent location, thereby also expanding the stent being carried by the balloon. The balloon is then deflated, leaving the expanded stent in place within the vessel. Balloon expandable stents have the advantage of being expandable to the exact diameter of the vessel into which they are being introduced, thus allowing for precise sizing. However, these stents are susceptible to external compression after implantation that can lead to loss of rigidity of the stent and perhaps even collapsing.
A self-expanding stent, which generally has an expanded stent position as its relaxed state, is introduced in a collapsed state into the body vessel and, upon triggering or release of a particular mechanism, is allowed to expand into its expanded, relaxed position. Self-expanding stents, on the other hand, are less susceptible to external compression but will expand only to a preset diameter. If the stent is undersized in comparison to the host vessel, the risk of stent migration within the vessel exists. Oversizing the stent relative to the host vessel, in order to ensure that the stent remains in place within the vessel, unfortunately increases the trauma to the vessel wall.
An ideal stent would have the precise sizing capabilities of the balloon-expandable stents and the compression resistance of the self-expanding stents.
U.S. Pat. No. 5,423,885 (Williams) shows an expandable, balloon intravascular stent that resists collapsing to a smaller diameter once it has been expanded to a larger diameter. However, the Williams stent is prevented from collapsing due to a plurality of protrusions on its outer surface that engage the walls of the artery or vessel into which it is disposed, thereby locking the stent in the larger diameter. This stent thus avoids radial collapse and axial displacement of the stent by use of protrusions that are anchored into the walls of the host vessel. This stent has the severe drawback of causing trauma and potential damage to the host vessel wall. It is desirable to provide a stent that will achieve these results without damaging the walls of the host vessel.